This invention relates to a method for controlling the wheel slip by means of an electronically controllable automotive vehicle brake system, in which electric signals representing the rotational behavior of the wheels are generated. From these signals--by means of electronic circuits--control signals are obtained for an anti-lock-controlled and/or traction-slip-controlling braking action. By comparing the rotational behavior of the wheels, vehicle cornering is identified and taken into account in controlling the braking force. Circuit configurations for implementing the method are also within the scope of this invention.
Various methods and circuit configurations are already known which provide for electronic anti-lock control and traction slip control including hydraulic or pneumatic brake systems having electronically controllable valves inserted into the pressure medium paths. In such systems, as soon as a wheel becomes unstable or shows a lock-up tendency, the braking pressure will temporarily be kept constant or reduced and will be reincreased at the appropriate time by blocking the pressure medium paths and/or by opening a pressure path from the wheel brake to an unpressurized reservoir so as to effect anti-lock control in case of panicky braking action or in case of excessive brake actuation caused by the other reasons. The necessary valve control commands for operating the system are generated from electric signals representing the rotational behavior of the individual wheels which signals are processed by electronic circuits such as microcomputers. For traction slip control, similar systems meter braking pressure into the wheel brake of the driven wheel that is spinning or tending to spin. The system also provides for engine management by reducing the engine's driving force simultaneously with metering of the braking pressure. By means of a logical combination of the information present in the form of electric signals relating to the rotational behavior of the individual wheels, the vehicle velocity etc. and by means of observation of the reaction to the braking pressure variation etc., the control unit identifies the instantaneous road conditions, the value of the friction coefficient on the right and left sides of the vehicle disregards interferential factors and meters just enough braking pressure into the individual wheel brakes so as to maintain driving stability and to achieve an effective slowing-down within a short stopping distance.
When interpreting the measured data such as velocity, deceleration and acceleration of the individual wheels and when computing the required braking pressure, it must be considered that the rotational behavior and the reaction of the wheels to pressure changes can have different causes and, thus, the measured data can be ambiguous. For instance, a high wheel slip or the lagging behind of one wheel in respect of the vehicular velocity or reference velocity can indicate an imminent lock-up. However, it also might be possible that the vehicle is in a narrow curve or is cornering wherein the wheels on the inside of the curve, in particular the turned front wheel, are moving more slowly. In order to tell one situation from the other and to be able to react appropriately to them, it is already known to insert a cornering identification device, such as that disclosed in published German Patent Specification No. P 31 27 302, into a circuit configuration of the type in question for vehicles with traction control. By means of the additional circuitry, a signal characteristic representing cornering is obtained by evaluating the sensor signals of the two non-driven wheels. To enhance driving stability in this known circuit, the driving torque of the automotive vehicle engine is reduced during cornering as soon as one of the drive wheels tends to spin and when, simultaneously, a threshold value of the vehicular velocity has been surpassed. In this system cornering identification is based only on measuring the difference in speed between the two front wheels. Without any further measurements, therefore, an unstable front wheel could incorrectly indicate that the vehicle is cornering. Thus, concerning identification is excluded during a controlled braking operation.
A circuit configuration for a slip-controlled brake system having a cornering identification circuit which adds the slip of the two wheels of one vehicle side at a time and compares them with the slip sum of the wheels on the other vehicle side is disclosed in German Published Patent Application DE-Offenlegungsschrift No. P 34 13 738. In this system, as soon as the difference in the slip sums of the two vehicle sides exceeds a limit value; there will be a temporary change in the selection criteria such as from "select-low operation" to "select-high operation" and, hence, a change in braking pressure variation. In this manner, braking pressure control will be adapted to the varying conditions of straight-forward driving and of cornering so as to maintain driving stability and steerability of the vehicle in all situations. In this circuit configuration, however, it is also impossible to exclude wrong interpretations of the measured results, for instance, in case of differences in the wheel velocities due to different scrub radii.